Post on 24-Mar-2018
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Lewis Base-catalyzed Conjugate Reduction and Reductive Aldol Reaction of α,β-
Unsaturated Ketones Using Trichlorosilane
Masaharu Sugiura, Norimasa Sato, Shunsuke Kotani and Makoto Nakajima
Faculty of Medical and Pharmaceutical Sciences, Kumamoto University,
5-1 Oe-honmachi, Kumamoto 862-0973, Japan
Supplementary Information
Table of Contents
General Methods S-1
Solvents and Chemicals S-2
General Procedure for Conjugate Reduction S-2 1H NMR Analysis of Reduction of Benzalacetone with Trichlorosilane S-5
General Procedure for Reductive Aldol Reaction S-6 1H and 13C NMR Spectra of All New Compounds S-10
HPLC Traces of the Optically Active Products S-15
References S-17
General Methods
Melting points (Mp) are uncorrected. 1H and 13C NMR spectra were measured in CDCl3
with JEOL JNM-ECX400 spectrometer. Tetramethylsilane (TMS) (δ = 0 ppm) and CDCl3 (δ
= 77.0 ppm) served as internal standards for 1H and 13C NMR, respectively. Infrared spectra
were recorded on JASCO IR Report-100. Mass spectra were measured with JEOL JMS-
DX303HF mass spectrometer. Optical rotations were recorded on JASCO P-1010
polarimeter. High-pressure liquid chromatography (HPLC) was performed on JASCO P-980
and UV-1575.
Thin-layer chromatography (TLC) analysis was carried out using Merck silica gel plates.
Visualization was accomplished with UV light, phosphomolybdic acid and/or anisaldehyde.
Column chromatography was performed using Kanto Chemical Silica Gel 60N (spherical,
neutral, 63-210 μm).
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All reactions were performed under argon atmosphere using oven- and heating gun-dried
glassware equipped with a rubber septum and a magnetic stirring bar.
Solvents and Chemicals
Dry dichloromethane (dehydrated) was purchased from Kanto Chemical and stored over
4Å MS prior to use. All other solvents were purified based on standard procedures.
Trichlorosilane was purchased from Tokyo Kasei Kogyo (TCI) and used without further
purification. (S)-BINAP dioxide (BINAPO) was prepared by oxidation of (S)-BINAP with
hydrogen peroxide in acetone. 1 All other chemicals were purified based on standard
procedures.
Starting materials: (E)-4-Phenyl-3-buten-2-one (benzalacetone, 1a), (E)-1,3-diphenyl-2-
propen-1-one (chalcone, 1b), α- and β-ionone (1e and 1f) and (R)-(+)-pulegone (1k) were
obtained commercially and used after purification by standard procedures. (E)-1-Phenyl-2-
buten-1-one (1c) was prepared by oxidation of (E)-1-phenyl-2-buten-1-ol with MnO2.
Dibenzylideneacetone (1d), 2-benzylidenecyclohexanone (1i) and 2,6-
dibenzylidenecyclohexanone (1j) were prepared by aldol condensation.2 2-Methyl-1-phenyl-
2-propen-1-one (1h) was prepared by Mannich reaction of propiophenone with
hexamethylenetetramine and acetic anhydride. (E)-1,3-Diphenyl-2-methyl-2-propen-1-one
(1g) was prepared according to the literatures.3
General Procedure for Conjugate Reduction
To a solution of a Lewis base [HMPA, Ph3P=O (20 mol%) or (S)-BINAPO (10 mol%)]
and an enone (1.0 or 0.5 mmol) in dry dichloromethane (2 or 1 mL) was added dropwise
trichlorosilane (1.5~3 M CH2Cl2 solution, 2 equiv) at 0 °C. The reaction was monitored by
TLC analysis. For the reactions of less reactive enones, the temperature was raised to
ambient temperature. After the starting material was consumed or no significant change was
observed, the reaction was quenched with sat. aqueous NaHCO3 (3 mL). After addition of
ethyl acetate (10 mL), the mixture was stirred for 1 h, filtered through a Celite pad with ethyl
acetate and extracted with ethyl acetate (3×). The combined organic layers were washed with
brine (1×), dried over anhydrous MgSO4, filtered, evaporated and purified by silica gel
column chromatography (hexane/ethyl acetate = 20/1~8/1) to give a reduction product.
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4-Phenylbutan-2-one (2a) (Table 1, entry 1)
According to the general procedure, the reaction of benzalacetone (1a) (145.6
mg) with HMPA and HSiCl3 (0 °C, 30 min) gave the reduction product (127.7
mg, 87% yield). The product was identified in comparison with a commercially available
sample.
1,3-Diphenylpropan-1-one (Table 1, entry 2)
According to the general procedure, the reaction of chalcone (1b) (208.3 mg)
with HMPA and HSiCl3 (0 °C, 30 min) gave the reduction product (191.7 mg,
91% yield). The product was identified in comparison with a commercially available sample.
1-Phenylbutan-1-one (butyrophenone) (Table 1, entry 3)
According to the general procedure, the reaction of (E)-1-phenyl-2-buten-1-one
(1c) (74.8 mg) with HMPA and HSiCl3 (0 °C, 30 min) gave the reduction
product (54.3 mg, 72% yield). The product was identified in comparison with a commercially
available sample.
(E)-1,5-Diphenyl-1-penten-3-one (Table 1, entry 4)
According to the general procedure, the reaction of dibenzylideneacetone
(1d) (111.7 mg) with HMPA and HSiCl3 (0 °C, 60 min) gave the reduction
product (89.7 mg, 80% yield). Spectroscopic data were consistent with the literature data.4
4-(2,6,6-Trimethyl-2-cyclohexen-1-yl)butan-2-one (Table 1, entry 5)
According to the general procedure, the reaction of α-ionone (1e) (192.2
mg) with HMPA and HSiCl3 (0 °C, 4 h then rt, 2 h) gave the reduction
product (180.7 mg, 93% yield). Spectroscopic data were consistent with the
literature data.4
4-(2,6,6-Trimethyl-1-cyclohexen-1-yl)butan-2-one (Table 1, entry 6)
According to the general procedure, the reaction of β-ionone (1f) (192.3
mg) with HMPA and HSiCl3 (0 °C, 30 min) gave the reduction product
(182.2 mg, 94% yield). Spectroscopic data were consistent with the
literature data.4
Me
O
Ph
Ph
O
Ph
Ph
O
Me
O
PhPh
Me
O
Me
MeMe
Me
O
Me
MeMe
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(±)-1,3-Diphenylbutan-1-one (Table 1, entry 7)
According to the general procedure, the reaction of 1,3-diphenyl-2-methyl-2-
propen-1-one (1g) (112.0 mg) with HMPA and HSiCl3 (0 °C, 5 h then rt, 19 h)
gave the reduction product (83.4 mg, 74% yield). Spectroscopic data were consistent with the
literature data.5
2-Methyl-1-Phenylpropan-1-one (isobutyrophenone) (Table 1, entry 8)
According to the general procedure, the reaction of 2-methyl-1-phenyl-2-propen-
1-one (1h) (65.8 mg) with HMPA and HSiCl3 (0 °C, 9 h then rt, 17 h) gave the
reduction product (39.7 mg, 60% yield). The product was identified in
comparison with a commercially available sample.
2-Benzylcyclohexanone (Table 1, entry 9)
According to the general procedure, the reaction of 2-benzylidenecyclohexanone
(1i) (92.6 mg) with HMPA and HSiCl3 (0 °C, 30 min) gave the reduction product
(83.5 mg, 89% yield). The product was identified in comparison with a
commercially available sample.
2-Benzylidene-6-benzylcyclohexanone (Table 1, entry 10)
According to the general procedure, the reaction of 2,6-
dibenzylidenecyclohexanone (1j) (137.5 mg) with HMPA and HSiCl3
(0 °C, 40 min) gave the reduction product (114.0 mg, 82% yield). Slightly
yellow oil; 1H NMR (400 MHz, CDCl3) δ 1.50-1.65 (m, 2H), 1.83-2.00 (m, 2H), 2.58-2.73 (m,
3H), 2.94-3.04 (m, 1H), 3.40 (apparent q, J = 8.9 Hz, 1H), 7.18-7.24 (m, 3H), 7.26-7.36 (m,
3H), 7.36-7.42 (m, 4H), 7.46 (t, J = 2.1 Hz, 1H); 13C NMR (100 MHz, CDCl3) δ 22.8, 28.4,
29.2, 36.9, 51.2, 126.2, 128.49, 128.58, 129.4, 130.3, 135.4, 135.9, 137.5, 140.3, 203.4;
HRMS (FAB): calcd for C20H21O (M+H+) 277.1592, found 277.1590; Elemental Analysis
calcd (%) for C20H20O: C 86.92, H 7.29; found: C 86.73, H 7.41.
(5R)-2-Isopropyl-5-methylcyclohexanone (Table 1, entry 11)
According to the general procedure, the reaction of (R)-(+)-pulegone (1k)
(151.7 mg) with HMPA and HSiCl3 (0 °C, 4 h then rt, 16 h) gave the reduction
O
PhPh
Ph
O
Ph
Me
Ph
O
Me
Me
O
Ph
O
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product (128.6 mg, 84% yield) as a diastereomeric mixture (trans/cis = 1.7/1). Spectroscopic
data were consistent with a commercially available sample [(–)-menthone] and the literature
data.4
(S)-1,3-Diphenylbutan-1-one (Scheme 2)
According to the general procedure, the reaction of 2-methyl-1-phenyl-2-
propen-1-one (1g) (111.8 mg) with (S)-BINAPO and HSiCl3 (0 °C, 20 h) gave
the reduction product (109.7 mg, 97% yield) with 97% ee (S). Spectroscopic data were
consistent with the reported data and the absolute configuration was assigned in comparison
of the optical rotation with the literature value.5 Colourless liquid; [α]23D +14.7 (c 1.005,
CCl4) for 97% ee (S) [lit.5 [α]25D –13.5 (c 1.00, CCl4) for 82% ee (R)]; HPLC (a combination
of CHIRALPAK AD-H and AS-H, 0.46 cmø × 25 cmL each, hexane/2-propanol = 39/1, flow
rate 1.0 mL/min, UV detection at 254 nm) tR = 14.4 min (S), 15.8 min (R).
1H NMR Analysis of Reduction of Benzalacetone with Trichlorosilane
To a solution of benzalacetone (27.5 mg) and Ph3P=O (10.5 mg) in CD2Cl2 (0.6 mL) in a
NMR tube capped with a rubber septum was added trichlorosilane (38 μL). Gradual
formation of the corresponding (Z)-trichlorosilyl enolate was observed by 1H NMR analysis.
After 4 h, almost the starting material (>95%) was consumed. The geometry of the enolate
was assigned based on a NOESY analysis of the reaction mixture (Chart 1).
(Z)-2-(Trichlorosilyloxy)-4-phenyl-2-butene 1H NMR (400 MHz, CD2Cl2, internal standard: solvent residual peak = 5.32
ppm) δ 2.35 (s, 3H), 3.41 (d, J = 7.3 Hz, 2H), 4.95 (t, J = 7.3 Hz, 1H), 7.15-
7.24 (m, 3H), 7.24-7.33 (m, 2H).
Ph
O
Ph
Me
Me
OSiCl3
Ph
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Chart 1. The NOESY spectrum of the trichlorosilyl enolate derived from benzalacetone.
General Procedure for Reductive Aldol Reaction
To a solution of a Lewis base [HMPA, Ph3P=O (20 mol%) or (S)-BINAPO (10 mol%)],
an enone (0.5 mmol) and an aldehyde (0.6 mmol, 1.2 equiv) in dry dichloromethane (2 mL)
was added dropwise trichlorosilane (ca. 3 M CH2Cl2 solution, 2 equiv) at the indicated
temperature. The reaction was monitored by TLC analysis. After the enone was consumed or
no significant change was observed, the reaction was quenched with sat. aqueous NaHCO3 (3
mL). After addition of ethyl acetate (10 mL), the mixture was stirred for 1 h, filtered through
a Celite pad with ethyl acetate and extracted with ethyl acetate (3×). The combined organic
layers were washed with brine (1×), dried over anhydrous MgSO4, filtered, evaporated and
purified by silica gel column chromatography (hexane/ethyl acetate = 20/1~3/1) to give the
corresponding aldol product.
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2-Benzyl-1,3-diphenyl-3-hydroxypropan-1-one (Table 2, entry 2)
According to the general procedure, the reaction of chalcone (1b) (104.2 mg)
and benzaldehyde (61 μL) with Ph3P=O and HSiCl3 (0 °C, 4 h) gave the aldol
product (123.3 mg, 78% yield) as a diastereomeric mixture (50:50).
Spectroscopic data were consistent with the literature data.6,7
2-Benzyl-3-hydroxy-3-(4-methoxyphenyl)-1-phenylpropan-1-one (Table 2, entry 3)
According to the general procedure, the reaction of chalcone (1b)
(104.2 mg) and p-anisaldehyde (73 μL) with Ph3P=O and HSiCl3 (0 °C,
4 h) gave the aldol product (120.0 mg, 69% yield) as a diastereomeric
mixture (52:48). Colourless viscous oil; IR (film on NaCl, cm–1) 3450, 1670, 1610, 1593,
1578, 1511, 1247, 1175, 1035, 700; 1H NMR (400 MHz, CDCl3) δ 2.79 (dd, J = 13.3, 5.5 Hz,
0.48H), 2.99 (dd, J = 13.3, 8.9 Hz, 0.48H), 3.10 (dd, J = 13.6, 3.7 Hz, 0.52H), 3.17 (dd, J =
13.6, 10.1 Hz, 0.52H), 3.30 (brs, 0.52H), 3.36 (brs, 0.48H), 3.72 (s, 1.56H), 3.73 (s, 1.44H),
3.98-4.10 (m, 1H), 4.91 (brd, J = 6.4 Hz, 0.48H), 5.02 (d, J = 5.0 Hz, 0.52H), 6.80 (d, J = 9.4
Hz, 1.04H), 6.82 (d, J = 9.4 Hz, 0.96H), 6.95-7.43 (m, 10H), 7.49 (d, J = 7.3 Hz, 1.04H), 7.66
(d, J = 7.3 Hz, 0.96H); 13C NMR (100 MHz, CDCl3) δ 33.9, 36.5, 55.0, 55.1, 55.8, 73.7, 75.3,
113.6, 113.8, 126.00, 126.24, 127.35, 127.40, 128.06, 128.14, 128.19, 128.24, 128.30, 128.86,
128.92, 132.84, 132.89, 133.72, 134.67, 137.37, 138.08, 138.57, 139.26, 158.82, 159.00,
204.6, 205.5; HRMS (FAB): calcd for C23H22O3Na (M+Na+) 369.1467, found 369.1474.
2-Benzyl-3-hydroxy-3-(4-nitrophenyl)-1-phenylpropan-1-one (Table 2, entry 4)
According to the general procedure, the reaction of chalcone (1b) (104.6
mg) and p-nitrobenzaldehyde (90.9 mg) with Ph3P=O and HSiCl3 (0 °C,
4 h) gave the aldol product (130.7 mg, 72% yield) as a diastereomeric
mixture (74:26). Colourless solid; Mp = 122-126 °C; IR (film on NaCl, cm–1) 3460, 1670,
1592, 1516, 1343, 698; 1H NMR (400 MHz, CDCl3) δ 2.96 (dd, J = 13.8, 4.1 Hz, 0.26H),
3.04 (dd, J = 13.5, 7.8 Hz, 0.74H), 3.11 (dd, J = 13.5, 7.3 Hz, 0.74H), 3.17 (dd, J = 13.8, 10.3
Hz, 0.26H), 3.86 (d, J = 1.8 Hz, 0.26H), 4.00-4.12 (m, 1H), 4.38 (d, J = 8.2 Hz, 0.74H), 4.99
(dd, J = 8.2, 4.6 Hz, 0.74H), 5.20 (brd, J = 1.8 Hz, 0.26H), 6.92 (d, J = 7.1 Hz, 0.52H), 6.99-
7.10 (m, 0.78H), 7.14-7.36 (m, 5.70H), 7.41-7.50 (m, 2.48H), 7.54-7.61 (m, 1.04H), 7.66 (d, J
= 7.6 Hz, 1.48H), 8.07 (d, J = 8.7 Hz, 1.48H), 8.14 (d, J = 8.7 Hz, 0.52H); 13C NMR (100
Ph
O OH
OMePh
Ph
O OH
NO2Ph
Ph
O
Ph
OH
Ph
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MHz, CDCl3) δ 33.4, 36.4, 53.6, 54.6, 72.9, 73.6, 123.5, 126.3, 126.67, 126.76, 127.09,
128.05, 128.21, 128.32, 128.43, 128.58, 128.64, 128.83, 128.95, 133.51, 133.75, 136.62,
136.72, 137.8, 138.3, 147.03, 147.10, 149.0, 150.3, 204.4, 205.0; HRMS (FAB): calcd for
C22H20O4 (M+H+) 362.1392, found 362.1385.
2-Benzyl-1,5-diphenyl-3-hydroxypentan-1-one (Table 2, entry 5)
According to the general procedure, the reaction of chalcone (1b) (104.1
mg) and hydrocinnamaldehyde (79 μL) with Ph3P=O and HSiCl3 (rt, 24 h)
gave the aldol product (33.1 mg, 19% yield) as a diastereomeric mixture
(52:48). Colourless viscous oil; IR (film on NaCl, cm–1) 3450, 1667, 1590, 1575, 1490, 1445,
693; 1H NMR (400 MHz, CDCl3) less polar isomer (major): δ 1.63-1.82 (m, 2H), 2.54-2.63
(m, 1H), 2.79-2.90 (m, 1H), 3.07 (dd, J = 13.8, 6.9 Hz, 1H), 3.11 (dd, J = 13.8, 7.8 Hz, 1H),
3.41 (d, J = 9.2 Hz, 1H), 3.70-3.84 (m, 2H), 7.05-7.26 (m, 10H), 7.40 (apparent t, J = 7.8 Hz,
2H), 7.54 (apparent t, J = 7.3 Hz, 1H), 7.79 (apparent d, J = 8.0 Hz, 2H); polar isomer
(minor): δ 1.76-1.88 (m, 1H), 1.90-2.03 (m, 1H), 2.64-2.74 (m, 1H), 2.76-2.92 (m, 2H), 3.10
(dd, J = 13.7, 5.0 Hz, 1H), 3.15 (dd, J = 13.7, 9.2 Hz, 1H), 3.78 (ddd, J = 9.2, 5.0, 3.7 Hz, 1H),
3.94-4.00 (m, 1H), 7.05-7.21 (m, 8H), 7.24-7.33 (m, 4H), 7.46 (apparent t, J = 7.3 Hz, 1H),
7.62 (apparent d, J = 7.4 Hz, 2H); 13C NMR (100 MHz, CDCl3) less polar isomer: δ 32.6,
36.5, 38.2, 52.0, 71.9, 125.8, 126.5, 128.23, 128.33, 128.39, 128.52, 128.70, 129.1, 133.6,
137.3, 138.7, 141.8, 206.4; polar isomer: δ 32.4, 33.6, 36.5, 53.2, 71.5, 126.0, 126.2, 128.3,
128.4, 129.0, 133.2, 137.2, 139.4, 141.6, 205.1; HRMS (FAB): calcd for C24H25O2 (M+H+)
345.1855, found 345.1865.
2-Ethyl-1,3-diphenyl-3-hydroxypropan-1-one (Table 2, entry 6)
According to the general procedure, the reaction of (E)-1-phenyl-2-buten-1-one
(1c) (73.4 mg) and benzaldehyde (61 μL) with Ph3P=O and HSiCl3 (0 °C, 4 h)
gave the aldol product (88.8 mg, 70% yield) as a diastereomeric mixture (39:61).
Spectroscopic data were consistent with the literature data.7
4-Hydroxy-3-(2,6,6-trimethyl-1-cyclohexen-1-yl)methyl-4-phenylbutan-2-one (Table 2,
entry 7) According to the general procedure, the reaction of β-ionone (1f)
(81.4 mg) and benzaldehyde (61 μL) with Ph3P=O and HSiCl3 (0 °C, 5 h)
Ph
O OH
Ph
Ph
Ph
O
Ph
OH
Me
O
Ph
OH
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gave the aldol product (82.6 mg, 65% yield) as a diastereomeric mixture (78:22).
Spectroscopic data were consistent with the literature data.7
2,2-Dimethyl-1,3-diphenyl-3-hydroxypropan-1-one (Table 2, entry 8)
According to the general procedure, the reaction of 2-methyl-1-
phenylpropenone (1h) (73.5 mg) and benzaldehyde (61 μL) with HMPA and
HSiCl3 (rt, 24 h) gave the aldol product (50.2 mg, 39% yield). Spectroscopic data were
consistent with the literature data.8
(3R,4R)-4-Hydroxy-3-(2,6,6-trimethyl-1-cyclohexen-1-yl)methyl-4-phenylbutan-2-one
(Scheme 2) According to the general procedure, the reaction of β-ionone (1f)
(96.5 mg) and benzaldehyde (120 μL, 2 equiv) with (S)-BINAPO and HSiCl3
(–78 °C, 21 h) gave the syn-aldol product (101.0 mg, 67% yield, 96% ee) and
the anti-aldol product including impurities (5.3 mg, 0% ee). The
diastereomers were separable by column chromatography. The diastereoselectivity was
determined by 1H NMR analysis of the crude reaction mixture. Spectroscopic data were
consistent with the reported data6 and the absolute configuration was assigned in comparison
of the optical rotation with the literature value.6
Syn-isomer: Colourless solid; [α]20D +56.9 (c 1.01, CHCl3) for 58% ee (3R,4R) [lit6 [α]20
D –
57.7 (c 1.0, CHCl3) for 75% ee (3S,4S)]; HPLC (a combination of two CHIRALPAK AD-H,
0.46 cmø × 25 cmL each, hexane/2-propanol = 29/1, flow rate 1.0 mL/min, UV detection at
254 nm) tR = 17.8 min (3S,4S), 21.3 min (3R,4R).
Anti-isomer: HPLC (a combination of two CHIRALPAK AD-H, 0.46 cmø × 25 cmL each,
hexane/2-propanol = 29/1, flow rate 1.0 mL/min, UV detection at 254 nm) tR = 18.4 min, 20.0
min.
Me
O
Ph
OH
Ph
O
Ph
OH
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1H and 13C NMR Spectra of All New Compounds
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HPLC Traces of the Optically Active Products
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